1. Field of the Invention
The present invention generally relates to beam welding techniques. More particularly, this invention relates to a method of electron beam welding (EBW) in which two beams are projected to simultaneously perform both the welding operation and a post-weld heat treatment to reduce strain age cracking of the weld joint.
2. Description of the Related Art
High temperature iron, cobalt and nickel-based superalloys are widely used to form hot section components of gas turbine engines, including combustors and turbine vanes (nozzles) and blades (buckets). Circumstances exist where superalloy components are preferably or necessarily fabricated by welding. For example, components having complex configurations, such as turbine midframes, shroud support rings and steam turbine nozzle assemblies (boxes), can be more readily fabricated by welding castings together. Various techniques have been developed for welding superalloys. Tungsten inert gas (TIG) and plasma transferred arc (PTA) techniques are widely used in manual welding operations. For more demanding applications, such as weld joints having high aspect ratios, laser beam and electron beam (EB) welding processes have been developed. As known in the art, electron beam welding involves directing a beam of high-energy electrons on a joint between articles held in a vacuum. Electron beam welding techniques are particularly well suited for producing weld joints having high aspect ratios, as electron beam welding yields the deepest penetrations of any beam process, with very high aspect ratios of about ten to fifty being readily achieved. EB weld machines of about 30 to 50 kW capacity are not uncommon for this purpose.
Components for the hot sections of gas turbines are required to survive extended periods at high temperatures and stresses without excessive deformation. For this reason, these components are frequently made from nickel-base superalloys, which achieve good high temperature strength through precipitation hardening of {circumflex over (1)}3xc3xa2xc2x72 (gamma prime) and {circumflex over (1)}3xc3xa2xc2x72xc3xa2xc2x72 (gamma double prime) phases. The precipitation of these phases frequently exhibits xe2x80x9cC-curvexe2x80x9d kinetics. When stages of the precipitation process are plotted on temperature vs. time axes, as represented in FIG. 1, the resulting curve is C-shaped and exhibits a xe2x80x9cnosexe2x80x9d at the temperature at which precipitation is most rapid. C-curve behavior is typical for nucleation-and-growth type processes such as precipitation.
Superalloys capable of surviving very high temperatures are often designed to contain a high volume fraction of gamma prime. For these alloys, precipitation at temperatures near the nose of the C-curve is very rapid. When components made from precipitation-hardened alloys are welded, gamma prime and gamma double prime phases are dissolved in and near the weld (in the heat-affected zone, or HAZ). When the component later experiences high temperatures near the nose of the C-curve (the xe2x80x9caging rangexe2x80x9d), these phases can precipitate out again. This strengthening process can take place more rapidly than the relaxation of residual stresses which remain from welding. The weld and surrounding area are thus incapable of accommodating the strains required to relieve the residual stresses, and the weld or HAZ cracks. This phenomenon is known as strain age cracking.
One approach which has been advanced to prevent strain age cracking is to heat the weldment and surrounding area to the particular alloy""s stress-relief temperature at a rate which is sufficiently rapid so that the strengthening phases do not have time to precipitate out. On the time-temperature curve of FIG. 1, this approach follows the upper heating curve that avoids the nose of the C-curve. However, as represented by the lower curve in FIG. 1, this solution is impractical when using traditional heating methods for relatively large components having significant thermal mass. Alternatives to post-weld stress relief have been developed to facilitate welding alloys that are prone to strain age cracking. For example, the Welding Institute (TWI), Cambridge, Great Britain, has developed a proprietary process for diverting a portion of an electron beam of an EB welder, enabling two separate beams to be independently directed on a component. The TWI process has been particularly developed to reduce the incidence of strain age cracking through two mechanisms. A first is to use the diverted beam to pre-heat the area to be welded ahead of the welding beam. Cracking is lessened due to the lower strength of the material at these temperatures, as well as due to the smaller temperature differential between the molten weld and the surrounding part. The second mechanism is to use the diverted beam to heat the component around the location of the welding beam. This technique is said to effect a thermal strain distribution that results in the weld being in a state of compressive residual strain when complete, reducing the incidence of cracking during post-weld heat treatment.
The present invention provides a method of forming a welded assembly, wherein an electron beam is used to form a weldment that joins two or more articles to form the assembly. The method is particularly directed to projecting a second electron beam onto the weldment in the wake of the welding beam, and in a manner that reduces strain-age cracking in the weldment.
The electron beam welding method of this invention generally comprises placing together two or more articles to define at least one contact surface interface therebetween. According to a preferred aspect of the invention, the articles are formed of alloys containing at least one precipitation-strengthening phase, such as a nickel-based superalloy containing gamma prime and/or gamma double prime precipitates. A first electron beam is caused to travel along the interface to form a weldment that joins the articles together, while a second electron beam is selectively directed at a portion of the weldment behind the first electron beam. According to another preferred aspect of the invention, the first and second electron beams are formed by splitting a third electron beam. The precipitation-strengthening phase is dissolved by the first electron beam so as to be substantially absent in the weldment. The second electron beam is at a sufficient power density and directed on the weldment an appropriate distance from the first electron beam so that the weldment is heated to a stress-relief temperature of the alloy and at a rate sufficient to avoid precipitation of the precipitation-strengthening phase. In this manner, the electron beam welding method of this invention results in the weldment and surrounding area being stress-relieved almost immediately after the weldment is formed, resulting in a welded assembly that is resistant to strain age cracking.
Other objects and advantages of this invention will be better appreciated from the following detailed description.